Vertical stacking

This is the general way stacking is thought of, with one credential combined with another to progress toward a higher credential.
Example: High school diploma > Associate degree > Bachelor’s degree

Horizontal stacking

In this type of stacking, a student earns a variety of credentials in related fields that, when taken collectively, prepare them for a specific type of job.
Example: Certificate in CompTIA (IT) + Certification Microsoft Certified Solutions Expert (MCSE) + Certification in Cisco Certified Network Associate (CCNA) = job as IT systems administrator.

Value-added stacking

This pertains to adding an area of expertise to an existing degree in order to be better prepared for a specific type of job.

Source: Salt Lake Community College